fix(#136): mobile-responsive wallet modals with bottom-sheet drawer

[Emmanard]

Summary

• Bottom-sheet on mobile: on viewports < 640 px the wallet modal now renders as a vaul Drawer sliding up from the bottom (rounded-t-2xl, drag handle, max-h-[90vh], pb-[env(safe-area-inset-bottom)] for iPhone home-bar)
• Desktop unchanged: the existing centred <Dialog> is untouched behind an isMobile guard
• A11y / Radix safety: introduced WalletModalHeader that renders DialogHeader/DialogTitle/DialogDescription on the dialog path and semantic <h2>/<p> on the sheet path — avoids running Radix Dialog context expectations inside a Drawer
• Single source of truth: WalletModalViews is shared by both shells so all four flows (connect, install, connecting, network-warning) live in one place
• Touch targets: install <a> rows min-h-[56px] + active: state; Drawer close button min-h-11 min-w-11 (44 px); primary buttons already h-12
• Responsive classes: p-4 sm:p-6, text-xl sm:text-2xl, w-16 sm:w-20 across all four views
• NetworkWarningView: Refresh / Continue Anyway row is now flex-col sm:flex-row so buttons stack on narrow screens
• New hook: hooks/use-is-mobile.ts — hydration-safe (false on SSR, real value post-mount via matchMedia)

Files changed

File	Change
hooks/use-is-mobile.ts	New — SSR-safe useIsMobile() hook
components/Wallet/WalletModal.tsx	Drawer/Dialog shell, shared views, header helper, responsive classes, touch targets

vaul was already a project dependency (used in navbar.tsx). No new packages added.

Test plan

• Open wallet modal on desktop (~1280 px) — centred Dialog renders, all four views transition correctly
• Open wallet modal on mobile emulation (~390 px) — bottom-sheet slides up, drag handle visible, close button tap target ≥ 44 px
• Verify connect → connecting → success flow on both breakpoints
• Verify install view links open correctly on mobile
• Verify NetworkWarningView buttons stack vertically at < 640 px
• Run Lighthouse mobile audit on any page that opens the wallet modal; confirm no new tap-target or viewport warnings
• Check iPhone Safari (safe-area inset) — sheet content should clear the home bar

Closes #136

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